Check your lower radiator hose and make sure it is not collapsing when you are having the heat problem. I have had that happen and it restricts the flow through the engine and back out to the top of the radiator. I did a flush and fill, new stat and cap only to find the hose collapsing. Just an idea and easy to check.

Yup...already checked that one. There's good circulation; both hoses (new) are full and piping hot. I make sure that every time I have the system changed / topped up that it's run cap-off for 15-20 mins to bleed the air out, too. I'm getting great circulation...it's the heat that's the issue...